Probably a couple hundred give or take. I love the Pepper's ghost illusion and usually do a couple of them every year. I have used 6'x8' plexi in the past but I have used regular glass the last couple of years because I needed 8'x8' sheets. Not fun moving and storing heavy glass but it makes a great effect.
Plexi makes a good effect too. Just make sure you support it so it is straight, otherwise your ghost will be distorted.
Jim H

salvage yards,buy some old sliding glass doors,they also have tempered glass.ive found huges ones before but i chose to have 2 small peppers ghosts instead of 1 large.the glass is like 8 ft by 4 ft on the ones we use

Vertically probably would be the most used position simply because it gives an opportunity to create a life-sized image with it's feet on the floor, possibly able to come and "GETCHA!"
It would also depend upon what and how much space you have to work with. If verticle is already filled up, maybe a horizontal space above the average line of sight would work? Like a G h o s t!!! flying passed near the ceiling?